Spyware
is a type of malware (malicious software) installed on computers that collects
information about users without their knowledge. The presence of spyware is
typically hidden from the user and can be difficult to detect. Typically,
spyware is secretly installed on the user's personal computer. However, some
spyware such as key loggers may be installed by the owner of a shared,
corporate, or public computer on purpose in order to secretly monitor other
users.

Trojan horse,
is a standalone malicious program that does not attempt to infect files unlike a
computer virus. Trojan horses can make copies of themselves, steal information,
or harm their host computer systems.
* It is
recommended to use a stable anti virus and anti spam software in your PC